Plasma etching method and apparatus

ABSTRACT

A plasma etching method includes the steps of placing a sample having metal a wiring portion on a sample table in a vacuum vessel, evacuating the vacuum vessel to establish a reduced pressure in the vacuum vessel, introducing an etching gas into the vacuum vessel while continuing to evacuate the vacuum vessel to maintain the reduced pressure in the vacuum vessel, and generating a plasma from the etching gas under the reduced pressure in the vacuum vessel using radio-frequency power. The plasma etches the metal wiring portion, and a residue forms on the metal wiring portion during the etching of the metal wiring portion by the plasma. The method further includes the step of applying to the sample table a bias voltage which periodically changes between two different voltages during the etching of the metal wiring portion by the plasma to remove the residue from the metal wiring portion.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 07/735,668filed on Jul. 26, 1991, now U.S. Pat. No. 5,900,162, which is acontinuation of application Ser. No. 07/475,204 filed on Feb. 5, 1990,now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma etching method and apparatus,and more particularly to a plasma etching method and apparatus which arewell suited for etching the samples of semiconductor device substrates,etc.

2. Description of the Prior Art

In recent years, the microfabrication of patterns has rapidly proceededwith the heightened densities of integration of semiconductor devices.

As techniques for precisely etching a microscopic pattern of about 1 μm,it has heretofore been practised to control ion energy which is incidenton a sample. The techniques are known from Japanese Patent Laid-Open No.61-13625, Japanese Patent Publication No. 61-41132, and so on. Thesetechniques are claimed to be effective for the etching of themicroscopic pattern of about 1 μm and the formation of a film of highquality.

At present, as a technique for etching a pattern which is still minuterthan the conventional pattern of about 1 μm, there is known a process(hereinbelow, termed "low-temperature etching") wherein a sample holderis furnished with a cooling device, and a sample is cooled to atemperature not higher than 0° C. which is a minimum temperature ofwater and then etched as disclosed in for example, Japanese PatentLaid-Open No. 60-158627. According to this process, the reactionsbetween a solid and neutral particles such as radicals are suppressed,and the amount of side etching or lateral etching at a side wall can bemade very small without lowering an etching rate in the depth directionof the sample. The low-temperature etching is therefore claimed to beeffective for the etching of the microscopic pattern.

As references concerning the low-temperature etching of this type, thereare mentioned, for example, Japanese Patent Laid-Open Nos. 63-128630,63-115338, and 63-174322, all of which disclose cooling a sample to atemperature not higher than 0° C. which is a minimum temperature ofwater.

The first-mentioned technology of controlling the ion energy has thefeature that, when the ion energy is increased, sputter etching based onions takes place mainly, so an anisotropic etching process becomespossible. On the other hand, however, there arise the problems that thesample is heavily damaged due to ion bombardment and that the ratio ofselectivity of a material to be etched to the subbing layer thereofbecomes small.

To the contrary, when the ion energy is decreased, an etching processwhich suffers from no damage and which affords a high selectivity ratiobecomes possible. On the other hand, however, the amount of the sputteretching based on the ions decreases, and hence, the period of time ofetching lengthens, resulting in the problem that the period of time ofisotropic etching based on neutral particles such as radicals prolongsso as to side-etch the sample. It has accordingly been difficult toapply the technology to the etching process of a pattern which isfurther minified.

The low-temperature etching technology is very effective for etching thestill minuter pattern at a good anisotropy. The inventors' experimentfor a higher practicability, however, has revealed that a problem to bestated below is involved in the performance of the low-temperatureetching.

When the sample is cooled to the low temperature and then subjected tothe etching process, the reactions between the neutral particles and thesolid are suppressed, and the etching weakens. Thus, some materials tobe etched give rise to residues on surfaces to be etched in such amanner that a substance which ought to react with the material to beetched and then vaporize as a reaction product adheres and remains onthe surface to be etched, or that the material to be etched remainsunetched.

Related to the present invention are U.S. Pat. Nos. 4,622,094 and4,795,529.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, in a process based onlow-temperature etching, a plasma etching method and apparatus capableof an etching process which produces no residue, which is anisotropicand which affords a high selectivity.

The present invention consists in an apparatus wherein a sample iscooled to a temperature not higher than 0° C. and is subjected to anetching process with a gas plasma, comprising means for repeatedlychanging an acceleration voltage which accelerates ions in said gasplasma toward said sample, and also consists in a method wherein asample is cooled to a temperature not higher than 0° C. and is subjectedto an etching process with a gas plasma, comprising the step ofrepeatedly changing an acceleration voltage which accelerates ions insaid gas plasma toward said sample, whereby in a process based onlow-temperature etching, the invention realizes an etching processproducing no residue, being anisotropic and affording a highselectivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an arrangement diagram showing a plasma etching apparatuswhich is one embodiment of the present invention;

FIG. 2 is a diagram of the pattern of a control signal according to theapparatus in FIG. 1;

FIG. 3 is a diagram of the impressing pattern of an acceleration voltageaccording to the apparatus in FIG. 1;

FIG. 4 is an arrangement diagram showing a plasma etching apparatuswhich is the second embodiment of the present invention;

FIG. 5 is a diagram of the pattern of a control signal according to theapparatus in FIG. 4;

FIG. 6 is a diagram of the impressing pattern of an acceleration voltageaccording to the apparatus in FIG. 4;

FIG. 7 is an arrangement diagram showing a plasma etching apparatuswhich is the third embodiment of the present invention;

FIG. 8 is a diagram of the pattern of a supplied voltage according tothe apparatus in FIG. 7;

FIG. 9 is an arrangement diagram showing a plasma etching apparatuswhich is the fourth embodiment of the present invention;

FIG. 10 is a diagram of the impressing pattern of an accelerationvoltage according to the apparatus in FIG. 9;

FIG. 11 is an arrangement diagram showing a plasma etching apparatuswhich is the fifth embodiment of the present invention; and

FIG. 12 is an arrangement diagram showing a plasma etching apparatuswhich is the sixth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, an embodiment of the present invention will be described withreference to FIGS. 1 to 3.

Referring to FIG. 1, a microwave etching apparatus is shown as a plasmaetching apparatus in this case. The microwave etching apparatus isconstructed of a vacuum processing chamber in which a sample table ismounted, a waveguide which introduces a microwave into the vacuumprocessing chamber, a microwave source which generates the microwave,magnetic field generation means for generating a magnetic field in thevacuum processing chamber, a processing gas feed device which feeds aprocessing gas into the vacuum processing chamber, an evacuation devicewhich evacuates the vacuum processing chamber to reduce the internalpressure thereof to a predetermined pressure, a cooling device whichcools the sample table to a low temperature, and means for drawing ionsin a plasma into a sample, herein, bias application means for bestowinga bias voltage on the sample table.

The vacuum processing chamber is herein configured of a vessel 10 and adischarge tube 14. The vessel 10 has a flange 11 mounted on the outerperiphery of the upper part thereof, and is let open at the upper part.The discharge tube 14 is gastightly mounted on the flange 11 so as tocover the open part, thereby to define a space communicating with theinterior of the vessel 10. In this case, the discharge tube 14 is in asubstantially hemispherical shape. Herein, the vessel 10 is formed ofstainless steel, while the discharge tube 14 is formed of quartz.

The waveguide is herein configured of a waveguide 31 of rectangularcross section and a waveguide 32 of circular cross section. Outside thedischarge tube 14, the waveguide 32 is disposed surrounding thisdischarge tube 14. The waveguide 31 is joined to the waveguide 32, and amagnetron 30 being the microwave source is mounted on the end of thewaveguide 31 remote from the waveguide 32. Outside the waveguide 32,solenoids 40 being the magnetic field generation means are wound roundthis waveguide 32.

The processing gas feed device is herein configured of a flow controlvalve 51 and a processing gas source 50. The processing gas source 50 isconnected through the flow control valve 51 to a conduit 12 which isprovided in the flange 11 in this case.

The evacuation device is herein configured of a pressure regulator valve61 and a vacuum pump 60. The vacuum pump 60 is connected through thepressure regulator valve 61 to an exhaust port 13 which is provided inthe bottom of the vessel 10 in this case.

The sample table 20 is mounted on the bottom of the vessel 10 through aninsulator 23 so as to penetrate this bottom. Herein, the insulator 23 iselectrically insulating and exhibits a favorable thermal insulation. Thespace 15 is defined between the upper surface of the sample table 20 andthe envelope of the discharge tube 14. A wafer 24 being the sample canbe placed on the upper surface of the sample table 20 by atransportation device not shown, and it is arranged substantiallyhorizontally with its surface to be processed facing the space 15.

The cooling device is herein configured of a coolant feeder 70, and acoolant channel 21 formed in the sample table 20. The inlet and outletof the coolant channel 21 are provided at that end part of the sampletable 20 which is protruded out of the bottom of the vessel 10. Thecoolant feeder 70 is connected to the coolant channel 21 through acoolant feed pipe 71 as well as a coolant return pipe 72.

The bias application means is herein configured of a radio-frequencypower source 80, a controller 81 and a matching unit 82. One terminal ofthe radio-frequency power source 80 is connected to the sample table 20through the matching unit 82, and another terminal thereof is grounded.The controller 81 is connected to still another terminal of theradio-frequency power source 80.

The controller 81 functions to supply the radio-frequency power source80 with a control signal of radio-frequency power previously programmedin this case, and to change during the etching of the wafer 24 the valueof radio-frequency power which is to be supplied to the sample table 20by the radio-frequency power source 80.

Besides, numeral 16 designates an earth electrode which surrounds thesample table 20 in a manner to be spaced therefrom, and which isgrounded through the vessel 10 in this case. Numeral 22 indicates aheater which is interposed between the upper surface of the sample table20 and the coolant channel 21. The heater 22 is connected to an electricpower source device not shown. The electric power source device isadapted to control its output to the heater 22.

In the plasma etching apparatus thus constructed, the etching process ofthe wafer is performed as follows:

First, the internal space of the vessel 10 and the discharge tube 14 isevacuated by the vacuum pump 60 and the pressure regulator valve 61 soas to have its pressure reduced to a predetermined value. Subsequently,the wafer 24 (herein, one wafer) is transported into the space 15 by theknown transportation means not shown, and the transported wafer 24 isarranged on the upper surface of the sample table 20 with its surface tobe etched facing upwards. The arranged wafer 24 is held on the sampletable 20 by an electrostatic attraction device or a mechanical clamp,not shown.

Meanwhile, a coolant is fed into the coolant channel 21 through thecoolant feed pipe 71 by the coolant feeder 70, and the coolant comingout of the coolant channel 21 is recovered through the coolant returnpipe 72. Thus, the sample table 20 is cooled, and the wafer 24 is cooledthrough the sample table 20. On this occasion, the wafer 24 iscontrolled to a predetermined temperature by the heater 22. Herein, thewafer 24 is held at a temperature not higher than 0° C. at which thevapor pressure of a reaction product becomes lower than the pressure ofan etching gas within the space 15. Incidentally, on this occasion, thecontrollability of the wafer temperature is made better still bysupplying a heat transfer gas such as helium gas between the wafer 24and the sample table 20.

Simultaneously, the etching gas at a predetermined flow rate isintroduced from the processing gas source 50 into the evacuated space 15under the reduced pressure through the flow control valve 51 as well asthe gas conduit 12. Part of the etching gas introduced into the space 15is exhausted by the vacuum pump 60. Thus, the pressure of the space 15is adjusted to the predetermined etching pressure.

Subsequently, a microwave which is at 2.45 GHz in this case isoscillated by the magnetron 30. The oscillated microwave is propagatedthrough the waveguides 31 and 32, and is caused to enter the dischargetube 14. In addition, the solenoids 40 are energized in a predeterminedamount so as to generate a magnetic field. Thus, the magnetic field isestablished in the discharge tube 14, the etching gas in the dischargetube 14 is excited by the action of the electric field of the microwaveand the established magnetic field, and a plasma is developed in thespace 15.

The surface to be etched of the wafer 24 held on the sample table 20 atthe predetermined temperature is subjected to the etching process by thedeveloped plasma. On this occasion, radio-frequency power which is at 2MHz herein is supplied to the sample table 20 by the radio-frequencypower source 80 during the etching process of the wafer 24. Besides, theoutput of the radio-frequency power from the radio-frequency powersource 80 is changed in accordance with a control signal from thecontroller 81. By way of example, this output is changed periodicallywithin certain limits.

As an example of the etching process on this occasion, there will bedescribed the case of the processing of the wafer 24 in which a subbinglayer is an oxide film, a material to be etched is an Al--Cu--Si alloyfilm, and a mask is of a photoresist. As the etching gas, a chloric gaswhich does not contain a gas for forming a side-wall protective film isemployed. The temperature of the wafer 24 is -10° C. The radio-frequencypower is at levels of 90 W and 40 W in this case, and it is periodicallychanged every predetermined time t₁, herein every second, by a controlsignal from the controller 81 as illustrated in FIG. 2. Thus, theradio-frequency power which is periodically changed every time intervalt₁ as illustrated in FIG. 3 is supplied to the sample table 20, and biasvoltages V₁ and V₂ are consequently generated. Ions in the plasma aredrawn toward the wafer 24 by the bias voltages, whereby the etchingprocess is performed. In an instance, when the wafer 24 was etched inthis way, the selectivity ratio of the film to be etched to the subbingoxide film was 20, and that of the same to the photoresist was 3.5,these selectivity ratios being high. Further, the etching was free fromside etching and produced no residue. In FIG. 2, a time t₀ is the pointof time at which the etching is ended.

By the way, in a case where the temperature of the wafer 24 was thenormal temperature (about 20° C.) and where the level of thehigh-frequency power was 90 W, the film to be etched could be etchedanisotropically and without any appreciable residue in such a mannerthat the etching rate was 1000 nm/min., and that the selectivity ratioof the film to be etched to the subbing oxide film was 13, while theselectivity ratio thereof to the photoresist was 2.5. These selectivityratios relative to the subbing oxide film and the photoresist wereunsatisfactory.

In this regard, the temperature of the wafer 24 was set at -10° C. inorder to attain an anisotropy, and the radio-frequency power was loweredto 40 W in order to enhance the selectivity ratios. In this case, theselectivity ratios to the subbing oxide film and to the photoresist wererespectively enhanced to 25 and 5. However, a residue appeared underthese conditions.

As described above, the sample table 20, and in turn, the wafer 24, iscooled and the power of the radio-frequency power source 80 is changedduring the etching process, thereby to realize the etching process ofthe wafer 24 which is favorable in producing no residue, beinganisotropic and affording high selectivity ratios.

This fact will be considered more. Several kinds of reaction productsare formed during the etching of the wafer 24. For the purpose ofetching the wafer 24 at a good anisotropy, the wafer temperature needsto be set at the temperature of one of the reaction products exhibitingthe lowest vapor pressure. For this reason, some substances will remainas residues without being sufficiently etched. The residue appears onthe side wall and bottom surface of the material to be etched. Theresidue on the bottom surface of the material to be etched is removed bythe ions in the plasma. However, the residue on the side wall of thematerial to be etched is not satisfactorily removed when the amount ofentrance of the ions in the plasma is small.

The ions in the plasma are drawn toward the wafer 24 by the biasvoltage, and are caused to enter the wafer 24. In this case, the biasvoltage is generated by supplying the radio-frequency power to thesample table 20. Therefore, the bias voltage is raised by enlarging theradio-frequency power, thereby to increase the amount of entrance of theions. Thus, the residue on the side wall of the material to be etched isalso removed. Since, however, the entrance energy of the ions is alsoenlarged by raising the bias voltage, the ratio of selectivity of thematerial to be etched to the subbing layer or the resist becomes small.

In this regard, the period of time during which the residue is removedby the high bias voltage and the period of time during which the etchingprocess is performed with the low bias voltage, namely, withoutdecreasing the selectivity ratio are alternately switched over bychanging the level of the radio-frequency power as in this embodiment.It will therefore be possible to realize the etching process producingno residue, being anisotropic and affording high selectivity ratios.

When the etching process of the wafer 24 has ended in this way, theoscillation of the microwave by the magnetron 30, the energization ofthe solenoids 40, the operation of the radio-frequency power source 80,the delivery of the control signal from the controller 81 and theintroduction of the gas into the space 15 are all stopped at that time.Thereafter, the wafer 24 subjected to the etching process is transportedout of the space 15 by the transportation means not shown. Besides,another wafer 24 to be processed is transported into the vacuumprocessing chamber as stated before, and it is subjected to an etchingprocess as in the foregoing.

According to the embodiment thus far described, in a process based onlow-temperature etching, the level of radio-frequency power to besupplied to a sample table 20 is periodically changed, whereby a biasvoltage changes periodically and the amount of ions in a plasma changesappropriately, to alternately carry out the removal of a residue on asurface to be etched and the etching of the surface at a highselectivity, so that an etching process producing no residue, beinganisotropic and being highly selective can be performed.

Now, the second embodiment of the present invention will be describedwith reference to FIGS. 4 to 6.

FIG. 4 shows a microwave plasma etching apparatus. In this figure, thesame members as in FIG. 1 are indicated by identical numerals, and theyshall be omitted from description. The point of difference of theembodiment in FIG. 4 from the embodiment in FIG. 1 is that a D.C. powersource 83 is comprised together with a radio-frequency power source 80,and that the output of the D.C. power source 83 is controlled by acontroller 81a in this case. Incidentally, numeral 84 designates aradio-frequency blocking unit. One terminal of the D.C. power source 83is connected to a sample table 20 through the radio-frequency blockingunit 84, and another terminal is grounded. The controller 81a isconnected to still another terminal of the D.C. power source 83.

In the apparatus thus constructed during the etching process of a wafer24, predetermined radio-frequency power is supplied to the sample table20 by the radio-frequency power source 80, while at the same time, aD.C. voltage is supplied to the sample table 20 by the D.C. power source83. Besides, on this occasion, the output of the D.C. voltage from theD.C. power source 83 is changed according to a control signal from thecontroller 81a. Herein, the control signal which is periodically changedevery time interval t₁ as illustrated in FIG. 5 is delivered from thecontroller 81a to the D.C. power source 83. Thus, bias voltages V₁ andV₂ which are periodically switched over every time interval t₁ asillustrated in FIG. 6 are applied to the sample table 20. Aradio-frequency voltage of predetermined level is supplied insuperposition on the bias voltages. In this way, the etching process isperformed similarly to the foregoing embodiment in FIG. 1.

According to the second embodiment thus far described, the bias voltagecan be periodically changed by the D.C. power source 83 and thecontroller 81a, so that the same effect as in the foregoing embodimentis attained.

Now, the third embodiment of the present invention will be describedwith reference to FIGS. 7 and 8.

FIG. 7 shows a microwave plasma etching apparatus. In this figure, thesame members as in FIG. 1 are indicated by identical numerals, and theyshall be omitted from description. The point of difference of theembodiment in FIG. 7 from the embodiment in FIG. 1 is that analternating current generation source 86 is comprised together with aradio-frequency power source 80, and that the output of the alternatingcurrent generation source 86 is controlled by a controller 81b in thiscase. Incidentally, numeral 85 designates a synthesis unit whichsynthesizes the outputs of the radio-frequency power source 80 and thealternating current generation source 86. One terminal of thealternating current generation source 86 is connected to a sample table20 through the synthesis unit 85, and another terminal is grounded. Thecontroller 81b is connected to still another terminal of the alternatingcurrent generation source 86.

In the apparatus thus constructed, during the etching process of a wafer24, radio-frequency power is supplied to the sample table 20 by theradio-frequency power source 80, while at the same time, an A.C. voltageis supplied to the sample table 20 by the alternating current generationsource 86. Besides, on this occasion, the frequency and voltage of theA.C. voltage to be delivered from the alternating current generationsource 86 are changed according to a control signal from the controller81b. Herein, a radio-frequency voltage is supplied to the sample table20 in superposition on the periodically changing A.C. waveform asillustrated in FIG. 8. In this way, the etching process is performedsimilarly to the foregoing embodiment in FIG. 1.

According to the third embodiment thus far described, theradio-frequency power is periodically changed by the alternating currentgeneration source 86 and the controller 81b, and the bias voltageapplied to the sample table 20 is periodically changed, so that the sameeffect as in the foregoing embodiment in FIG. 1 is attained.

Now, the fourth embodiment of the present invention will be describedwith reference to FIGS. 9 and 10.

FIG. 9 shows a microwave plasma etching apparatus. In this figure, thesame members as in FIG. 1 are indicated by identical numerals, and theyshall be omitted from description. The point of difference of theembodiment in FIG. 9 from the embodiment in FIG. 1 is that a sampletable 20 is grounded, while a grid electrode 87 is provided over thesample table 20, and that a D.C. power source 83 is connected to thegrid electrode 87 and has its output controlled by a controller 81a inthis case. One terminal of the D.C. power source 83 is connected to thegrid electrode 87, and another terminal is grounded. The controller 81ais connected to still another terminal of the D.C. power source.

In the apparatus thus constructed, during the etching process of a wafer24, a minus D.C. voltage is supplied to the grid electrode 87 by theD.C. power source 83. Besides, on this occasion, the output of the D.C.voltage from the D.C. power source 83 is changed according to a controlsignal from the controller 81a. Herein, the control signal which isperiodically changed every time interval t₁ is delivered from thecontroller 81a to the D.C. power source 83. Thus, acceleration voltagesV₁ and V₂ which are periodically switched over every time interval t₁ asillustrated in FIG. 10 are supplied to the grid electrode 87. In thisway, the etching process is performed similarly to the foregoingembodiment in FIG. 1. In this embodiment in FIG. 9, the accelerationvoltage which is supplied to the grid electrode 87 is employed as meansfor drawing ions in a plasma toward the wafer 24.

According to the fourth embodiment thus far described, the accelerationvoltage to be supplied to the grid electrode 87 is periodically changedby the D.C. power source 83 and the controller 81a, and the amount ofthe ions in the plasma to be drawn toward the wafer 24 can be regulatedas in the foregoing embodiment in FIG. 1, so that the same effect as inthe foregoing embodiment is attained.

Although each of the first to fourth embodiments has been described asto the apparatus in which the plasma for the etching process isgenerated using the microwave, such an present invention is notrestricted to the apparatus. By way of example, the invention is alsoapplicable to an RIE apparatus of the parallel plate electrode type asshown in FIG. 11 or FIG. 12.

The fifth embodiment of the present invention will be described withreference to FIG. 11.

FIG. 11 shows a plasma etching apparatus of the parallel plate type. Inthis figure, the same members as in FIG. 4 are indicated by identicalsymbols, and they shall be omitted from description. The point ofdifference of the embodiment in FIG. 11 from the embodiment in FIG. 4 isthat parallel plate type electrodes consisting of a sample table 20 anda counter electrode 93 are disposed in a vacuum vessel 90, and that aplasma is generated by the electrodes. The counter electrode 93 isgrounded. Incidentally, numeral 91 designates a gas conduit, numeral 92an exhaust port, and numeral 94 a plasma generation space definedbetween the electrodes.

In the apparatus thus constructed, radio-frequency power is supplied tothe sample table 20 by a radio-frequency power source 80 so as togenerate the plasma in the space 94, whereupon a wafer 24 is subjectedto an etching process. During the etching process, a D.C. voltage issupplied to the sample table 20 by a D.C. power source 83 as in thesecond embodiment in FIG. 4. Besides, on this occasion, the output ofthe D.C. voltage from the D.C. power source 83 is changed according to acontrol signal from a controller 81a.

In this way, a bias voltage changing periodically is supplied to thesample table 20 as in the second embodiment, and the etching process isperformed similarly to the embodiment in FIG. 1. Thus, the same effectas that of the embodiment in FIG. 1 is attained.

The sixth embodiment of the present invention will be described withreference to FIG. 12.

FIG. 12 shows a plasma etching apparatus of the parallel plate type. Inthis figure, the same members as in FIGS. 9 and 11 are indicated byidentical symbols, and they shall be omitted from description. The pointof difference of the embodiment in FIG. 12 from the embodiment in FIG.11 is that the same acceleration voltage as in the fourth embodiment isused as means for drawing ions in the plasma toward a wafer 24.

Thus, likewise to the fourth embodiment, the sixth embodiment attainsthe same effect as that of the embodiment in FIG. 1.

According to the present invention described above, there is broughtforth the effect that, in a process based on low-temperature etching, anetching process producing no residue, being anisotropic and being highlyselective can be carried out.

What is claimed is:
 1. A plasma etching method comprising the stepsof:placing a sample having a metal wiring portion on a sample table in avacuum vessel; evacuating the vacuum vessel to establish a reducedpressure in the vacuum vessel; introducing an etching gas into thevacuum vessel while continuing to evacuate the vacuum vessel to maintainthe reduced pressure in the vacuum vessel; generating a plasma from theetching gas under the reduced pressure in the vacuum vessel usingradio-frequency power, the plasma etching the metal wiring portion, aresidue forming on the metal wiring portion during the etching of themetal wiring portion by the plasma; and applying to the sample table abias voltage which periodically changes between two different voltagesduring the etching of the metal wiring portion by the plasma to removethe residue from the metal wiring portion.
 2. A plasma etching methodcomprising the steps of:placing a sample having an aluminum alloyportion on a sample table in a vacuum vessel; adjusting a temperature ofthe sample to a temperature at which a residue is formed on the aluminumalloy portion during etching of the aluminum alloy portion by a plasma;evacuating the vacuum vessel to establish a reduced pressure in thevacuum vessel; introducing an etching gas into the vacuum vessel whilecontinuing to evacuate the vacuum vessel to maintain the reducedpressure in the vacuum vessel; generating a plasma from the etching gasunder the reduced pressure in the vacuum vessel using radio-frequencypower, the plasma etching the aluminum alloy portion, a residue formingon the aluminum alloy portion during the etching of the aluminum alloyportion by the plasma; and applying to the sample table a bias voltagewhich periodically changes between two different voltages to remove theresidue from the aluminum alloy portion.